1. Field of the Invention
The present invention relates generally to the field of batteries, especially to the field of recombinant or valve regulated lead acid (xe2x80x9cVRLAxe2x80x9d) batteries and, more specifically, to a method for producing battery separators containing fusible binders and fibers, preferably, glass fibers. Such separators have been manufactured by a method which involves producing, by a wet paper making process, sheets of glass fibers and fusible fibers, and heating the fusible fibers to cause them to bond the sheet into a more or less stable condition. The sheets are sometimes formed into desired shapes, e.g., pockets into which battery plates can be inserted, and the edges are heated so that fusible fibers therein weld the sheets in the formed shapes. In use, the separators are positioned between the positive and negative plates of batteries, where they prevent metal conduction, but do not prevent the migration of ions, between the plates. Separators for VRLA batteries, in service, have unfilled voids through which oxygen can migrate between the positive and negative plates.
2. Description of the Prior Art
U.S. Pat. No. 4,265,985, granted May 5, 1981 to O""Rell et al., discloses the production of battery separator from a slurry composed of 30 to 70 weight percent of polyolefin fibers, 15 to 30 weight percent of a particulate siliceous filler and 1 to 35 weight percent of long fibers which are polyester paper making staple fibers, acrylic paper making staple fibers, glass fibers, or mixtures of polyester, acrylic and glass fibers. The slurry is cast on the screen of papermaking apparatus and is pressed between opposed hard rolls before being transferred to an open meslh metal belt and passed through an unheated oven and then over a plurality of cans with surfaces heated to about 270xc2x0 F. The patent suggests that some bonding results from melting of the polyolefin fibers at fiber junctions.
A Japanese patent application published Jun. 19, 1982, under Number JP 57098975, discloses a separator produced from a slurry of glass fibers having an average diameter of 0.5 xcexcm and polyethylene fibers having an average diameter of 20 xcexcm. The slurry is cast on a screen as a narrow belt, dehydrated and dried. The dried separator is cut to size, and folded to form a bag; high frequency welding is then used to fuse the edges of the bag. The polyethylene fibers can constitute from 25 to 50 percent by weight of the total fibers.
A paper entitled xe2x80x9cSealed Car Batteryxe2x80x9d which was published by Mitsushita in October, 1986, in an internal newsletter, discloses that a newly developed separator provides improved reliability and productivity because it is in pouch form, and that an improvement was sought xe2x80x9cby the addition of highly resilient PET resin fiber to the constituent material . . . xe2x80x9d The authors were aware, the paper also says, that ultrasonic welding could be used to make a pouch form from a thick mat separator, but xe2x80x9ca need to ratchet up the welding technology level was recognized.xe2x80x9d
U.S. Pat. No. 5,436,094, granted Jul. 25, 1995, assigned to Mitsui Petrochemical Industries, Ltd. (xe2x80x9cMitsuixe2x80x9d) discloses the production of a separator material xe2x80x9cwith melt adhesion bondsxe2x80x9d from a slurry containing 5 to 95 percent by weight of a synthetic pulp, and 5 to 50 percent by weight of a polymer binder. The patent also discloses that 40 percent by weight or less of other synthetic or inorganic fibers can be used, naming, as xe2x80x9cother synthetic or inorganic fibersxe2x80x9d, polypropylene, polyethylene, nylon, polyester, acrylic, Modacryl, glass, silica and rock wool fibers. The separator material is produced by dispersing in water and defiberizing, for example in a pulper, the synthetic pulp, binder and the auxiliary fibers, if used, refining the slurry, and feeding the refined slurry onto the wire of a Fourdrinier or other paper making machine. The web which forms on the wire is then sprayed with a surfactant and dehydrated by suction, by pressing, or both, and dried and heat treated to form melt-adhesion bonds xe2x80x9cof the binder among tile fibers.xe2x80x9d The use of what is called xe2x80x9cthe method of the hot air through typexe2x80x9d is disclosed as being operable for the heat treatment. This method is described as involving heating the dried web from above while withdrawing air from below as the web is being advanced on a xe2x80x9cmesh-likexe2x80x9d support, and heat treating by passing the heated web over a hot roll, through a hot oven, or through an infra-red heater, or by subjecting it to ultrasonic energy.
EPO 587 682 B1, granted Apr. 12, 1995, discloses another method for producing a similar product from a mixture of glass fibers and at least one type of synthetic fibers. The synthetic fibers can be (a) a mixture of at least two types of fibers with different melting points, a first type which softens when the sheet is manufactured (i.e., during a heat treatment step) and a second type which melts when the sheet is formed, or (b) compound fibers, which can be of the sheath core or side by side type, consisting of a first synthetic material which softens when the sheet is manufactured (i.e., during a heat treatment step) and a second synthetic material which melts when the sheet is welded. In both cases, a separator material is produced by suspending the fibers in acidified water, casting the suspension on the screen of a paper making machine, and drying and heat treating the fibers; fiber to fiber bonds are formed as a consequence of the melting which occurs during the final heat treating step and subsequent re-solidification of the lower melting fibers or of the lower melting synthetic material of the composite fibers. The separator sheet can be cut to size and stacked between alternate positive and negative battery plates to produce assemblies which are used in the production of batteries, or it can, as the reference states, be welded by high frequency or ultrasonic welding to form pockets which receive the plates of batteries from which such assemblies are produced. The reference also states that high frequency or ultrasonic welding is desirable because it provides resistant welding whereas hot seam welding is much or fragile.
When separator produced by the methods of U.S. Pat. No. 5,436,094 and of EPO 587 682 B1 is cut to size, for example in preparation for the production of a stack of negative and positive battery plates with the separator positioned between adjacent plates, or for the production of a welded, envelope-type separator that is closed on all but one side, and in which a battery plate is encased, some of the separator necessarily becomes scrap. Furthermore, it is sometimes necessary to scrap separator, as produced, because its physical properties are not within specifications. Because of the bonds formed during production of the separator by the prior art methods, it is difficult, if not impossible, to produce a slurry containing that scrap and additional fibers which can be cast on the screen of a paper making machine to produce non-woven sheets from which additional separator can be made. The instant invention is based upon the discovery that separator material can be produced from a slurry formed by dispersing in water and defiberizing, for example in a pulper, glass fibers and heat bonding fibers, e.g., sheath-core fibers with a polyethylene sheath and a polypropylene core, refining, and feeding the resulting slurry onto the wire of a Fourdrinier or other paper making machine; the web which forms on the wire can then be sprayed, if desired, with a surfactant, dehydrated by suction, by pressing, or both, and dried at a temperature sufficiently low that melt-adhesion bonds do not form among the fibers. The unbonded, non-woven sheet which results can be tested to make sure that it meets specifications and, if so, can be rolled, packaged and shipped to another location, for example, from a separator manufacturer to a battery manufacturer. If the sheet does not meet specifications, it can be slurried, either alone or with additional fibers, and the slurry can be fed onto the wire of a Fourdrinier or other paper making machine and processed as described above to produce unbonded, non-woven sheet, so that the material which did not meet specifications is recycled. The separator which met specifications and was shipped can be removed from the package and cut to size, in preparation for the production of a stack of negative and positive battery plates with the separator positioned between adjacent plates, or for the production of welded separator that is closed on all but one side, and in which a battery plate can be encased. Scraps which are formed when the separator is cut to size can be slurried with additional fibers, water and the like, to produce a new slurry which can be fed onto the wire of a Fourdrinier or other paper making machine and processed as described above to produce non-woven separator. The sized, unbonded separator is then heated to a temperature sufficiently high to soften the binder, after which it is cooled to a temperature sufficiently low that the binder resolidifies and forms a bonded separator by binding the fibers to one another at points of contact; the bonded separator is then used to produce the stacks of negative and positive plates with separator between adjacent plates which are required for battery production. Separator which is produced according to the method of the instant invention has a given wet strength as produced, which is usually measured in the machine direction, which is the direction the material moves on the paper making machine, and in the transverse direction which is across the web at a right angle to the machine direction. The wet strength remains the same until after the material is bonded, which, the instant invention contemplates, will be done at a battery-making facility. After bonding, the wet strength is substantially higher than the given wet strength in both the machine direction and in the transverse direction.
In the following examples, and elsewhere herein, the terms xe2x80x9cpercentxe2x80x9d and xe2x80x9cpartsxe2x80x9d refer to percent and parts by weight, unless otherwise indicated; m means meter or meters, mm means millimeter or millimeters, g means gram or grams, xcexcm means micrometer or micrometers, and all temperatures are in degrees C. These examples constitute the best modes presently contemplated by the inventor, but are presented solely to illustrate and disclose, and are not to be construed as limiting, the invention.